1. Field of the Invention
This invention relates to dielectric fluids. In one aspect the invention relates to dielectric fluids comprising a triglyceride of C8 and/or C10 fatty acids while in another aspect, the invention relates to dielectric fluids comprising triglycerides of C8 and C10 fatty acids in combination with C18:1 fatty acids and polyunsaturated fatty acids.
2. Description of the Related Art
Vegetable oil (VO) based dielectric fluids, e.g., transformer fluids, have been increasingly used in the power generation industry to replace mineral oil (MO) based dielectric fluids because of their environmental friendliness and high flash point to improve safety of transformer operation. However, the VO-based dielectric fluids have significantly higher viscosity than the MO-based dielectric fluids, which results in a poorer heat transfer operation using a VO-based dielectric fluid. Therefore, a market need exists for a reduced viscosity VO-based dielectric fluid to improve the heat transfer efficiency in the transformer, while retaining the benefits of a high flash point combined with low melting point and low amounts of polyunsaturated fatty acids in the triglycerides. Table 1 provides a comparison of unmet properties of various oils available from natural sources. In particular, corn oil, cottonseed oil, linseed oil, canola oil, safflower oil, sunflower oil, and soy oil have a high content of triglycerides of polyunsaturated fats>15% while castor, crambie, lesquerella, and olive have higher viscosity>33 cP and coconut oil and palm oil have high melting points.
TABLE 1Comparison of Properties of VariousOil Available from Natural SourcesMeltingPolyun-Viscosity (cp)Flash Pointpointsaturatedat 40 deg C.(deg C.)(deg C.)content (%)Castor>33>300Coconut25Corn>300>50%Cottonseed>50%Crambie>33JojobaNon triglyceride containing componentsLesquerella>33Linseed>50%Olive>33>−15Palm>−15Rapeseed>300>30%(Canola)Safflower>300>50%Sunflower>300>50%Soya>300>30%VeroniaFunctionalized triglycerides (epoxidized fatty acids)
Some of the conventional approaches to address this problem, and their associated disadvantages, include
1. Lowering the viscosity of VO-based dielectric fluid by blending it with lower viscosity fluids such as polyalphaolefins, synthetic polyol esters and polyglycerol fatty acid ester. However these approaches can lead to lowering of the flash point or to substituting with a non-natural based source;
2. Mixing the VO-based dielectric fluid with a diluent such as fatty acid alkyl ester, but this requires a diluent in excess of 10 weight percent (wt %) to reduce the viscosity of a canola oil to less than 33 centipoise (cP). However, this also results in lowering of the flash point;
3. Increasing the amount of unsaturation in the VO-based dielectric fluid lowers the viscosity of the fluid, but it also lowers the oxidation stability of the fluid (see U.S. Pat. No. 6,117,827); and
4. Increasing the amount of saturated C12-C16 triglycerides in the VO-based dielectric, but this also increases the melting point of the fluid.
Of continuing interest is a dielectric fluid that possesses a desired balance of properties, specifically a combination of low viscosity (≦33 cP at 40° C., ≦120 cP at 10° C.), high flash point (≧260° C., preferably ≧270° C.), and low melting point (−8° C. or less).